TL;DR
Cartier made the Devonshire black opal jewel in the late 1930s for Mary Cavendish, then Marchioness of Hartington. It can be worn as a necklace, a tiara, five clips, or a bodice ornament — and at Queen Elizabeth II’s coronation in 1953, Mary wore it as a necklace, just behind the Queen. It goes to auction at Sotheby’s Royal & Noble Jewels sale in Geneva on 11 November 2026, with a reported estimate of CHF 780,000 to CHF 1.1 million.
Most jewels are made to be worn one way. This one was made to be rearranged. As a necklace, it walked in the coronation procession of 1953. Fitted to its frame, it becomes a tiara. Taken apart, it becomes five brooches.
A Cartier Jewel for a Marchioness, Most Likely for a Coronation
Sotheby’s account places the creation firmly, then carefully hedges the occasion. Cartier created the jewel in the late 1930s for Mary Cavendish (1895–1988), then Marchioness of Hartington and later Duchess of Devonshire. “Most likely,” Sotheby’s writes, it was commissioned in the context of the 1937 coronation of King George VI, when Cartier London reached its creative pinnacle during the Art Deco period. The qualifier matters: as The Court Jeweller notes, it is not known whether Mary wore the newly completed jewel at the 1937 coronation itself.
Mary was born in 1895, the daughter of the fourth Marquess of Salisbury, and married Edward Cavendish, Marquess of Hartington, in 1917. In 1938 they became the tenth Duke and Duchess of Devonshire. The jewel was carried through loss: their eldest son, William, was killed in action in Belgium in 1944, four months after marrying Kathleen Kennedy, sister of John F. Kennedy, and the tenth Duke died in 1950. Widowed at fifty-five, Mary turned to royal service — and to the jewel, which Sotheby’s says she wore from the 1930s onward in all its transformations.
The Opals Came From a Tour, According to Family Tradition
The stones are told as family tradition, not documented fact. According to that tradition, the black opals were provided by the client. The Marquess and Marchioness had toured Australia and New Zealand in 1936–1937 — Edward was then Under-Secretary of State for Dominion Affairs — during which, Sotheby’s writes, they were presented with six exceptional opals, “Australia’s signature gem.” The book for the Victoria and Albert Museum’s Cartier exhibition gives a more personal version, quoted by The Court Jeweller: the gems, probably Australian and of exceptional quality, were supplied by Mary herself, a gift from her husband during the tour. No mine is named in either account, and none should be guessed.
That leaves a small puzzle for anyone counting. Sotheby’s tour account speaks of six exceptional opals presented to the couple, while the jewel as described in sale coverage is built around five detachable brooches, each centred on an oval black opal. This post does not resolve the difference into one total for the finished jewel: the six-opal figure belongs to the tour, and the five-brooch figure belongs to the construction.
One Jewel, Four Ways to Wear It
The design is where Cartier’s Art Deco confidence shows. The jewel transforms: necklace, tiara, five clips, and a devant-de-corsage fitting, worn at the front of a bodice. Haute Living’s preview describes the five detachable brooches — each centred on an oval black opal, set with old mine-cut, old European-cut, and single-cut diamonds, accented by baguettes, with a pear-shaped diamond at the front — plus the tiara fitting, the devant-de-corsage fitting for the three central brooches, and the fitted case stamped Cartier. One suite of stones, designed to be taken apart and reassembled for the occasion, the dress, and the light.
Mary used that freedom as intended, wearing the brooches on her jackets at events during the war years, when she chaired the Victoria League, promoting friendship across the Commonwealth; she received a CBE from George VI in 1946 for that work.
The Coronation of 1953: Worn as a Necklace, Not as a Tiara
The jewel’s most celebrated appearance is also its most commonly misdescribed one. On 2 June 1953, at the coronation of Queen Elizabeth II, Mary — by then Dowager Duchess of Devonshire and Mistress of the Robes, an office she held from 1953 to 1967 — walked just behind the Queen and her train-bearers, assisting with the quick-changes and rituals of the ceremony. She wore the opal and diamond jewel as a necklace. On her head she wore something else: the diamond-set Devonshire palmette tiara, an older family jewel. Sotheby’s calls the necklace one of the jewels most prominently featured during the ceremony, “second only to those of the Queen herself,” in a ceremony it notes was the first coronation filmed in its entirety in colour and televised.
That precision is the point of a convertible jewel. Mary chose the necklace form, paired with a tiara carrying the family’s longer history: the Cartier jewel supplied the colour and the modernity, and the palmette tiara supplied the lineage.
Why Is Black Opal So Expensive?
Part of the answer is optical. Precious opal shows play-of-colour — flashes of spectral colour that shift as the stone moves — because of its internal structure. As GIA’s field report from the Australian opal fields explains, precious opal is composed of tiny silica spheres packed in a grid, and light is broken into rainbow colours by the voids between them: spheres of about 0.20 microns diffract blue light, 0.25 microns green, and 0.32 microns red. In a black opal, that play-of-colour appears against a dark body tone, making the flashes look brighter than they would against a pale stone. Disordered silica gives common opal, with no play-of-colour — so the finest material is rare twice over.
The rest is scarcity and story. GIA notes that Australia supplies the world’s primary source of black and boulder opal, known there since the first recorded discovery of precious opal in the 1840s. Fine black opal with strong play-of-colour is uncommon even within that supply, and a suite of matched, exceptional stones is rarer still. Add a documented royal wearing, a Cartier making, and a design that does four jobs, and the price is no longer only for stones — the kind of stone story explored at Lumière Gems.
From a Museum Wall to a Film Premiere, Then to Geneva
The jewel did not retreat into a vault after 1953. It was shown in the Victoria and Albert Museum’s Cartier exhibition in London in 2025, and on 5 February 2026 director Emerald Fennell wore the necklace to the London premiere of her film adaptation of Wuthering Heights at the Odeon Theatre, Leicester Square. Sotheby’s treats the choice as apt: Fennell used jewellery throughout the film to reflect her heroine’s evolution, and a director that attentive to jewellery choosing a jewel with this history feels less like a loan than like casting.
The sale is Sotheby’s Royal & Noble Jewels sale in Geneva. Opulent Club’s preview puts the exhibition at the Mandarin Oriental, Geneva, from 6 to 10 November, daily from 10 a.m. to 6 p.m., ahead of the live auction on Wednesday, 11 November 2026 at 5 p.m. CET. Advance reports put the estimate at CHF 780,000 to CHF 1.1 million, about US$945,000 to US$1.3 million. Estimates are invitations, not valuations, and the result will say how the market weighs the parts of this jewel against each other: the stones, the making, and the two coronations attached to it — one most likely, one certain.
Mary Cavendish wore it as a necklace behind a queen, as brooches on a jacket in wartime, and as a tiara when the occasion called for it. A jewel that could do all of that, and still look inevitable in each form, did not need a single defining moment. It had a life instead — and in Geneva, that life passes, intact and still transformable, to whoever understands what they are buying.